This application relates generally to microelectromechanical systems, and more particularly to methods for affirming a switched status of MEMS-based devices.
In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (MEMS) and are desirable because they may be constructed with considerable versatility despite their very small size. One example of a MEMS component structure is a micromirror, which is generally configured to reflect light from at least two positions. Such micromirrors find numerous applications, including as parts of optical switches, display devices, and signal modulators, among others.
In many applications, such as may be used in fiber-optics applications, such MEMS-based devices may include hundreds or even thousands of micromirrors arranged as an array. Within such an array, each of the micromirrors should be accurately aligned with both a target and a source. Such alignment is generally complex and typically involves fixing the location of the MEMS device relative to a number of sources and targets. If any of the micromirrors is not positioned correctly in the alignment process and/or the MEMS device is moved from the aligned position, the MEMS device will not function properly.
MEMS devices provide for individual movement of each of the micromirrors. An example is provided in FIGS. 1A-1C illustrating a particular MEMS micromirror structure that may take three positions. The micromirror structure illustrated in FIGS. 1A-1C is of the torsion-beam type. Each micromirror includes a reflective surface 116 mounted on a micromirror structural film 112 that is connected by a structural linkage 108 to an underlying substrate 104. Movement of an individual micromirror is controlled by energizing actuators 124a and/or 124b disposed underneath the micromirror on opposite sides of the structural linkage 108. Hard stops 120a and 120b are provided to stop the action of the micromirror structural film 112.
Energizing the actuator 124a on the left side of the structural linkage 108 causes the micromirror to tilt on the structural linkage 108 towards that side until one edge of the micromirror structural film 112 contacts the left hard stop 120a, as shown in FIG. 1A. Alternatively, the actuator 124b on the right side of the structural linkage 108 may be energized to cause the micromirror to tilt in the opposite direction, as shown in FIG. 1B. When both actuators are de-energized, as shown in FIG. 1C, the micromirror returns to a static position horizontal to the structural linkage 108. In this way, the micromirror may be moved to any of three positions. This ability to move the micromirror provides a degree of flexibility useful in aligning the MEMS device, although the alignment complexity remains significant. Sometimes hard stops 120a and 120b are not provided so that the micromirror structural film 112 is in direct contact with the substrate 104.
For telecommunications applications, optical MEMS devices are typically enclosed within a sealed hermetic enclosure and surrounded by control electronics with accompanying software. A factor in maintaining a high level of reliability for the telecommunications system includes affirming that one or more particular mirrors has switched to the desired position when commanded to route a particular optical signal. One approach that has been proposed to accomplish this is to monitor and detect modulations in the optical power of an optical signal during switching. Other proposals have exploited the fact that in some applications each MEMS device is associated with a particular wavelength, permitting an optical interrogator to be installed for detecting that particular wavelength, and thereby determining that the MEMS device has been switched. A more direct nonoptical approach attempts to sense the MEMS device capacitively after switching, but such sensing is problematic without monolithic integrated on-chip electronics.
There is accordingly a need in the art for methods that permit affirming the switched status of MEMS-based devices.
Embodiments of the invention are thus directed to a MEMS device and a method for operating the MEMS device to determine whether it is in a select state. The select state is defined by a position of a moveable element, which is moved with electrostatic forces upon activation of an electrode. The moveable element may be conductive or semiconductive in different embodiments. The select state is detected with a sensing configuration that has first and second regions. The regions are generally separated such that they are electrically uncoupled unless the moveable element is in the position that defines the select state. In some embodiments, a detector may be provided to indicate whether the first and second regions are so coupled electrically.
The sensing configuration may be organized differently in a number of embodiments. For example, in one embodiment, the sensing configuration comprises a transistor, which may be a field-effect transistor or a bipolar junction transistor. Where the sensing configuration comprises a field-effect transistor, the first region comprises a source of the field-effect transistor and the second region comprises a drain of the field-effect transistor. The moveable element acts as a gate that couples the source and drain electrically when in the select state. Where the sensing configuration comprises a bipolar junction transistor, the first region comprises an emitter of the bipolar junction transistor and the second region comprises a collector of the bipolar junction transistor. The moveable element acts as a base that couples the emitter and collector electrically when in the select state.
In another embodiment, the first and second regions comprise first and second waveguide ports. The impedance between the waveguide ports is reduced when the moveable element is positioned so that the device is in the select state, and the reduced impedance may be detected.
In a further embodiment, the moveable element comes into contact with both the first and second regions when position so that the device is in the select state. A current in the completed circuit between the regions may be detected.
In certain embodiments, the MEMS device comprises a microstructure for steering light. The microstructure comprises a substrate and a structural linkage connected with the substrate to support the moveable element. The position of the moveable element orients a reflective coating off of which light may be reflected. The sensing configuration may be formed within the substrate.
A plurality of such MEMS devices may also be configured as an array, and such an array may be comprised by a wavelength router for optical applications. In one embodiment, the first region of each of the devices and the electrode of each of the devices are electrically coupled with a dynamic refresh driver. The second regions of the devices are electrically coupled with one another. Such a configuration limits the number of bond pads that need be included with the structure.